CNS Repopulation by Hematopoietic-Derived Microglia-Like Cells Corrects Progranulin deficiency

Abstract

Hematopoietic stem cell transplantation can deliver therapeutic proteins to the CNS through donor-derived hematopoietic cells that become microglia-like cells. However, using standard conditioning approaches, hematopoietic stem cell transplantation is currently limited by low and slow engraftment of microglia-like cells. We report an efficient conditioning regimen based on Busulfan and a six-day course of microglia depletion using the colony-stimulating factor receptor 1 inhibitor PLX3397. Combining Busulfan-myeloablation and transient microglia depletion results in robust, rapid, and persistent microglia replacement by bone marrow-derived microglia-like cells throughout the CNS. Adding PLX3397 does not affect neurobehavior or has adverse effects on hematopoietic reconstitution. Through single-cell RNA sequencing and high-dimensional CyTOF mass cytometry, we show that microglia-like cells are a heterogeneous population and describe six distinct subpopulations. Though most bone-marrow-derived microglia-like cells can be classified as homeostatic microglia, their gene signature is a hybrid of homeostatic/embryonic microglia and border associated-macrophages. Busulfan-myeloablation and transient microglia depletion induce specific cytokines in the brain, ultimately combining myeloid proliferative and chemo-attractive signals that act locally to repopulate microglia from outside the niche. Importantly, this conditioning approach demonstrates therapeutic efficacy in a mouse model of GRN deficiency. Transplanting wild-type bone marrow into Grn-/- mice conditioned with Busulfan plus PLX3397 results in high engraftment of microglia-like cells in the brain and retina, restoring GRN levels and normalizing lipid metabolism.

Evaluate the efficiency of Mr BMT in macrophage replacement of peripheral organs

“…The existing microglia of recipient mice will be replaced by microglia-like cell derived from bone marrow cells [11,12] . This strategy can be useful in cases where the resident microglia are dysfunctional and contribute to the development of neurodegenerative, such as Alzheimer's disease (AD) [13] , Parkinson disease (PD) [14] and Frontotemporal dementia (FTD) [15] . However, it is important to evaluate the potential effects of Mr BMT on peripheral tissue macrophages when considering this therapy for clinical 4 translation, since the approach used for microglia ablation also have significant effects on the tissue macrophages and monocyte populations [16][17][18].…”